This invention relates to a single mode optical fiber coupler capable of effecting substantially uniform coupling of light across a predetermined relatively broad range of wavelengths, as well as relating to the method of manufacture thereof.
It is desirable to employ single mode waveguides and coupling devices therefor because a straight-forward light path is provided therein. Hereinafter the terms "waveguide" and "fiber" shall be used interchangeably and refers to the core of the fiber surrounded by its cladding. A single mode waveguide or fiber is understood to be a waveguide which conducts or can conduct only a single mode of light. Typically, these waveguides are glass fibers which are very small in diameter. Further, the size of the core of these fibers is on the order of 5-10 microns or less. This compares with the relatively large core size of multimode fibers which are typically on the order of at least 50 microns and typically about 50-200 microns. Accordingly, it can be appreciated that the manufacture of couplers for single mode fibers is a difficult undertaking since, because of the relatively small fiber size, it is relatively easy to destroy the waveguide path or core and/or detrimentally affect it resulting in increased losses in transmission, the result of which is highly undesirable in the case of single mode fibers.
In accordance with the disclosure of copending application Ser. No. 833,635 now abandoned of Paul Kopera et al., which disclosure is specifically incorporated by reference herein, there is disclosed a method of making single mode fiber optic couplers. Typically, the fiber optic couplers disclosed therein are those of which are known as wavelength dependent fiber optic couplers. More specifically, by wavelength dependent is meant that the single mode couplers will operate to effect substantially uniform coupling between two fibers at very specific wavelengths, however, the results will be affected detrimentally if the wavelength of the light being employed is varied from the specifically desired wavelength.
In the field of fiber optics, there are three wavelengths ranges upon which coupling across the board range must be effected. The first length is typically between about 1200 nm-1500 nm. This range is of special interest in present technology because it is employed in long haul communications. The other ranges are typically between 700-900 nm and 400-600 nm.
In the prior art, it has been desirable to make fused single mode couplers which are wavelength independent, such that a number of different wavelengths can be transmitted through the fiber with little or no loss over a broad range. For a discussion of wavelength dependence of fused couplers, see Electronics Letters, Mar. 13, 1986, volume 22, Number 6 "Wavelength Dependence of Fused Couplers".
In the past, there have been devised several schemes for fabricating fused couplers which yield devices having splitting ratios substantially independent of wavelength. One such scheme is discussed in Electronics Letters, Aug. 15, 1985, volume 21, No. 17 "Wavelength-Flattened Fused Couplers". In this document, there is discussed a technique for fabricating fused couplers which yields devices having splitting ratios substantially independent of wavelength. The couplers are fabricated by using fibers of different diameter or profile, or by tapering one of two identical fibers more than the other. The devices are made by pretapering one fiber and then the tapered fibers were twisted with untapered fiber of constant diameter and the pairs heated and pulled to cause the fusing. A disadvantage with this process, however, is that it is difficult to taper one fiber and then hold the two fibers together to fuse since the fibers are fragile and can break. Further, because of the buffer on the fibers at the portions which are being held, there is a slippage effect and thus, reproducabilities with the process becomes difficult.